Comet 6x24 binoculars from the inside – what went wrong?

28 September 2017
Marcin Górko

2. Eyepieces

The binoculars feature non-adjustable but comfortable eyecups made of plastics which is, I suppose, a close relative of Bakelite. Their shape and the eye relief distance, amounting to 13.5 mm, provide proper comfort during observations allowing you to survey the whole field of view. At least it is true for people who don’t wear glasses; those with astigmatism who are forced to use them won’t be so happy. The eyecups won’t let them observe the whole field of view and after removing them there’s a risk of scratching the elements with the thread which fixes the eyecup to the tube. A ribbed individual focusing ring is the next part of the Comet eyepieces. It also features a ±6 dioptre scale. Here you hit the first snag: after setting the focus at infinity with a help of normal, healthy eye, no eyepiece shows „0” on the scale. The left eyepiece is set at +1 dioptre, the right one at about −2,5 dioptre. Mind you I set the focus in both of them with the same eye… Additionally the left eyepiece is distinctly bent to the left.

Comet 6x24 binoculars from the inside – what went wrong? - Eyepieces
Something is wrong with the assembly of eyepieces: when the dioptre scale index is set at the same distance from the focusing ring one scale shows −1 dioptre, and the second as much as −6 dioptre

Is this an explanation of fuzziness spread, described by Arek in his test? Fortunately the answer is no. The ribbed ring is attached to the eyepiece slide with three press bolts, every one of them 120° apart from the other. If you ease them the dioptre scale ring can be turned as you wish because the eyepiece remains immobile. It is very easy to “0” calibrate the binoculars with the focus set at infinity. What makes the eyepiece bent, though? It moves along the axis on its own and the ring’s diameter is a tad too big so there’s some slack between the ring and the case of the eyepiece. If the aforementioned three press bolts aren’t equally tight the ring will be set non-axially compared to the eyepiece. In that state exactly the Comet was sent from its factory…

Comet 6x24 binoculars from the inside – what went wrong? - Eyepieces
After unscrewing eyecups you gain access to three press bolts which allow you to calibrate the dioptre scale on the eyepiece. If you remove the bolts the focusing ring can be taken out of the eyepiece slide.

The optics of eyepiece is set inside a perfectly blackened aluminum tube. If you unscrew the ring from the bottom it is possible to pull out the elements. Eyepieces of the Comet are the “singlet-doublet-doublet” constructions. It is not a classic example of an wide angle Erfle or Koenig eyepiece which should be characterized by a field of view of 65°. Unfortunately the edges of the elements aren’t blackened at all. It causes dispersion of rays of light which don’t create images and, in turn, contrast decrease and brightening of the area around the exit pupil. Blackening the edges is, fortunately, an easy DIY task; it’s enough you have a black CD felt tip pen, some patience and a steady hand.

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Comet 6x24 binoculars from the inside – what went wrong? - Eyepieces
Eyepieces’ elements don’t have properly blackened edges; as a result contrast of images is decreased. The same eyepiece before and after blackening of edges.

During reassembling the eyepiece I discovered another slip-up of the producer: the diameter of optical elements and distance rings is a bit too small when compared to the inner diameter of the eyepiece tube. As a result the whole optics is a bit “loose” inside. In such a situation nobody, not even the producer, would be able to center the whole eyepiece system perfectly. It causes huge fuzziness on the edge of the field, one must add different in the left and in the right eyepiece … That’s how in practice looks the ‘cooperation’ between eyepiece optics specialists and engineers responsible for the framework. The optics specialists make eyepieces a bit too small so they fit the tubes without any problem; the engineers produce a bit too wide tubes. That way the assembly is a piece of cake but overall it’s like acting against your own interests.

Comet 6x24 binoculars from the inside – what went wrong? - Eyepieces
The same eyepiece before (left) and after the blackening (right). The effect is perhaps not especially pronounced in the photo but it helps with dampening unwanted light reflections during the observation.

Finally one last digression concerning the eyepieces: lack of a wide-angle eyepiece in a pair of binoculars with a small magnification ratio is a serious mistake. These instruments are constructed exactly to offer a wide field of view along a huge exit pupil. Leitz engineers knew about it while constructing their famous Amplivid 6x24; the Russians knew it too, building the KOMZ 6x24 but the Chinese don’t learned that lesson somehow. Pity. With the Erfle eyepiece system the Comet would offer a huge, sensibly corrected field of view, amounting to more or less 10.8°, and as such it would be a big market hit. It is also worth mentioning that, although the producer declare a field of view of 10.8° it is in reality just 9.1 degrees.